Hi Paul,
> lot and use 8' diameter shielded, tuned (160m) loops made of 1"
> hardline.--thought I would try, on my feedlines, some of the techniques
If the loop is shielded and if the shield is properly connected, it cures
the common mode problems.
What happens is the shield becomes the actual antenna element that receives
the signal (both electric and magnetic fields--so Faraday and Maxwell don't
roll over in their graves!). Since the element is grounded in the middle
(hopefully) of the bottom, it can be nearly perfectly balanced and common
mode currents will NOT excite the antenna at all even if they do flow up
the feedline!
If a loop is properly constructed, feedline decoupling should make no
change at all. Also, the problem could be direct coupling from the loop to
the source of your noise, rather than conduction of noise to the antenna
via the feedline.
The problem with the Pennant antenna, and some others, is they are not
perfectly balanced nor perfectly unbalanced. One end of the feedline is
closer to earth and other things than the other, and the degree of that
unbalance plus the common mode impedance affects the coupling. It is a very
complex problem with such antennas, and probably a lot of "luck" enters the
picture! I can't think of a way to predict the amount of coupling with even
somewhat reasonable accuracy.
> With each bead having an AL of 3900 mh/1000 turns my calculations show a
> reactance of about 2400 ohms for the choke.
You know of course AL changes with frequency..and it is really ALR, since
with 73 material Q is one at 2 MHz.
> Should I ground the shield of the loop?
I would, as long as it is there.
> If I go to untuned loops I could use them on more than one band. Is there
> anything published in the ham literature on the care and feeding of
untuned
> loops?
Nothing worthwhile.
> Is it enough just to put a good choke balun on the feedline where
> it attaches to the untuned loop?
That's all I do. It works very well for me.
> Is it practical, or desirable, to match
> the very low impedance of the loop to the coax?
I don't bother with that. It will improve signal level, but that isn't a
problem with a good preamplifier.
> Until I can find some rural acreage, I will continue to fight to improve
my
> receiving S/N in this noisy, electrically poluted environment--every db
helps.
You may just be too close to stuff. You are certainly in the near field of
everything in your lot. You might have to try a lot of things to get
something that works, since near field problems are esoteric problems.
73 Tom
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